Dual beam, or differential spectrophotometry involves the separation of a single beam of light into two beams, one of which is directed at a sample material or solution which may be contained in a "sample" container cell or cuvette, and the second of which is directed toward a reference material or solution which may be contained in a reference "standard" container cell or cuvetter containing only solvent or another known material. The light emitted from the two cells is then observed by one or more light detectors and compared as a function of wavelength. When the spectrofluorescence of a sample solution is to be compared with that of a reference solution the cells are arranged so that the light detector(s) receive only the fluorescent light caused by incidence of the two respective beams on the cells, and none of the excitation light directly incident upon the sample and reference cells. If, however, the light absorption properties of the sample solution are to be compared with the standard reference material, then the light incident on the two cells must be directed through the two materials so that absorption can take place, and then the unabsorbed light is directed to the light detector(s) for comparison as a function of wavelength.
Whether an apparatus is used as a differential spectrofluorometer or as a differential absorption spectrophotometer, it is important that the dual beam light paths, cells and number of reflections encountered should be as alike as possible so that no characteristic of the apparatus itself will affect the resulting comparison operation.
The present invention provides a dual beam spectrophotometer which can be converted very simply by the operator from a differential spectrofluorometer to a differential absorption spectrophotometer, and vice versa, merely by the insertion or removal of a reflective diagonal divider into or from the sample and reference container cells, or alternatively by the substitution of container cells having or not having reflective diagonals in place.